Method of producing lithium ion secondary battery

ABSTRACT

A method for producing lithium ion secondary batteries includes the steps of: (A) preparing an electrode sheet with lead-forming parts, (B) intermittently forming porous insulating layers containing an inorganic oxide filler and a binder on a surface of the electrode sheet excluding the lead-forming parts, (C) connecting a lead to each of the lead-forming parts, and (D) fabricating batteries by using the electrode sheet to which the leads are connected. The step B includes: the step of applying a slurry containing the inorganic oxide filler and the binder to the outer surface of a gravure roll, and transferring the slurry applied to the outer surface of the gravure roll on a surface of the electrode sheet that is being transported by a plurality of guide rolls excluding the lead-forming part; and the step of moving at least one selected from the gravure roll and the guide rolls to make the electrode sheet away from the gravure roll in the lead-forming part.

TECHNICAL FIELD

The present invention relates to a method for producing lithium ionsecondary batteries with improved safety against short-circuits, andmore specifically, to an effective method for intermittently formingporous insulating layers on a surface of an electrode sheet.

BACKGROUND ART

Lithium ion secondary batteries have a separator between the positiveelectrode and the negative electrode, the separator having the functionsof electrically insulating these electrode plates and further holding anelectrolyte. The separator for lithium ion secondary batteries iscurrently made of a microporous thin film sheet composed mainly ofpolyethylene. When such a thin film sheet is used, an internalshort-circuit may occur, since an active material that has fallen offfrom electrode plates in a manufacturing process of a battery or foreignmatter included in the battery may penetrate the thin film sheet.

In order to prevent the occurrence of internal short-circuits in lithiumion secondary batteries, and as an effective means for preventing theexpansion of short-circuiting, there has been a proposal to form aprotective film (porous insulating layer) containing insulatingparticles (inorganic oxide filler), such as alumina powder, and a resinbinder on the surface of the positive electrode or the negativeelectrode (Japanese Patent Publication No. 3371301). The porousinsulating layer is usually formed by applying a slurry containing aninorganic oxide filler and a resin binder on the electrode surface anddrying it.

Generally, the electrode for lithium ion secondary batteries is formedof an electrode plate in sheet form. Such an electrode sheet comprisesan electrode core member and an electrode material mixture carried oneach side of the electrode core member. The electrode sheet needs tohave a lead-forming part for connecting a current-collecting lead. Asthe lead-forming part, an exposed part of the electrode core member(current collector), i.e., a part having no electrode material mixtureapplied thereon, is usually utilized. It is common from the structuralrequirements of the battery that there is a location displacementbetween the part having no electrode material mixture applied thereon onone side of the electrode core member and the part having no electrodematerial mixture applied thereon on the other side thereof.

A typical method for applying an electrode material mixture to anelectrode core member is a method of intermittently applying a pastecontaining an electrode material mixture to an electrode core memberusing a die coater (Japanese Patent Publication No. 2842347). There hasalso been proposed a method of applying a paste containing an electrodematerial mixture to an electrode core member by means of a gravure rollwithout forming a part having no electrode material mixture appliedthereon (Japanese Laid-Open Patent Publication No. 2001-179151).However, this method requires peeling a part of the electrode materialmixture off the core member in order to form a part having no electrodematerial mixture applied thereon. There has also been proposed a methodin which a part of an electrode core member is masked with tape, a pastecontaining an electrode material mixture is applied to the electrodecore member, and thereafter the tape is peeled off. Further, there hasalso been proposed a method in which a part having no electrode materialmixture applied thereon is formed by clotting a part of an electrodematerial mixture applied to an electrode core member and then peeling itoff (Japanese Laid-Open Patent Publication No. Hei 10-247490).

DISCLOSURE OF INVENTION Problem that the Invention is to Solve

In the case of forming a porous insulating layer by applying a slurrycontaining an inorganic oxide filler and a resin binder to a surface ofan electrode sheet and drying it, the same method is also employed asthat employed when a paste containing an electrode material mixture isapplied to an electrode core member. However, when a porous insulatinglayer is formed on the surface of an electrode sheet having alead-forming part, the following problems arise.

First, when a slurry is intermittently applied to a surface of anelectrode sheet by means of a die coater, in principle, the existence ofthe part having no electrode material mixture applied thereon causes achange in the distance between the slit die and the surface of theelectrode sheet. As a result, the slurry cannot be applied in uniformthickness to the surface of the electrode sheet. Specifically, theslurry is not sufficiently applied, so that the underlying electrodematerial mixture is exposed in streaks and the porous insulating layerbecomes uneven. Such an uneven porous insulating layer cannot performits essential function, and in addition, it allows charge and dischargereactions to proceed unevenly, thereby becoming a cause of cycle lifedegradation.

Also, in the field of gravure printing using a gravure roll, since thereis no concept of intermittent application, a slurry containing aninorganic oxide filler and a resin binder is applied to the part havingno electrode material mixture applied thereon. Hence, a step of peelingthe slurry applied to the lead-forming part becomes necessary. However,such peeling step not only reduces production yields but also causes aproblem of residues of insulating material on the peeled surface. In thecase of peeling a paste containing an electrode material mixture appliedto an electrode core member, even if the material mixture is left on thelead-forming part, degradation of battery characteristics hardly occurs,since the electrode material mixture itself is conductive. However, ifthe inorganic oxide filler or the resin binder is left on thelead-forming part, the contact resistance between the lead and theelectrode core member increases, thereby resulting in degradation ofbattery characteristics.

Further, in the case of masking the lead-forming part with tape inadvance, a masking step and a tape-peeling step become necessary,thereby leading to a significant reduction in production yields.

Under such circumstances, the present invention proposes an effectivemethod of intermittently forming porous insulating layers containing aninorganic oxide filler and a binder on a surface of an electrode sheetexcluding lead-forming parts by using a gravure roll.

Means for Solving the Problem

The present invention relates to a method for producing lithium ionsecondary batteries, including the steps of: (A) preparing an electrodesheet with lead-forming parts, (B) intermittently forming porousinsulating layers comprising an inorganic oxide filler and a binder on asurface of the electrode sheet excluding the lead-forming parts, (C)connecting a lead to each of the lead-forming parts, and (D) fabricatingbatteries by using the electrode sheet to which the leads are connected.The step B includes: the step of applying a slurry comprising theinorganic oxide filler and the binder to the outer surface of a gravureroll, and transferring the slurry applied to the outer surface of thegravure roll on a surface of the electrode sheet that is beingtransported by a plurality of guide rolls excluding the lead-formingpart; and the step of moving at least one selected from the gravure rolland the guide rolls to make the electrode sheet away from the gravureroll in the lead-forming part.

The step A preferably includes the step of applying a paste comprisingan electrode material mixture to the outer surface of a gravure roll,and transferring the paste applied to the outer surface of the gravureroll on a surface of an electrode core member that is being transportedby a plurality of guide rolls.

At least a part of the outer surface of the gravure roll used in thestep (A) and/or the step (B) is preferably covered with ceramic.

In the step A, a part of the paste applied to the outer surface of thegravure roll is preferably scraped off by a blade without beingtransferred to the surface of the electrode core member. Also, in thestep B, a part of the slurry applied to the outer surface of the gravureroll is preferably scraped off by a blade without being transferred tothe surface of the electrode sheet.

In the step (A) and/or the step (B), the traveling direction of theouter surface of the gravure roll is preferably opposite to thetraveling direction of the electrode core member or the electrode sheet.

EFFECTS OF THE INVENTION

According to the present invention, porous insulating layers comprisingan inorganic oxide filler and a binder can be intermittently formed inuniform thickness on a surface of an electrode sheet having lead-formingparts. It is usually difficult to form porous insulating layers inuniform thickness on an electrode sheet having lead-forming parts (partshaving no electrode material mixture applied thereon).

Also, according to the present invention, the contact resistance betweena lead and an electrode core member in the lead-forming part does notbecome large. Further, since the porous insulating layer has a uniformthickness, its functions of preventing short-circuits and expansion ofshort-circuiting are enhanced, and the charge and discharge reactionsproceed uniformly. As a result, it is possible to provide a lithium ionsecondary battery with excellent cycle characteristics.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view showing an exemplary applicationdevice with a gravure roll;

FIG. 2 is a schematic sectional view of a negative electrode sheetaccording to Comparative Example 1;

FIG. 3 is a schematic sectional view of a negative electrode sheetaccording to Comparative Example 2; and

FIG. 4 is a schematic sectional view of a negative electrode sheetaccording to Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, an exemplary method for producing a lithium ionsecondary battery of the present invention is described.

First, the step A of producing an electrode sheet with lead-formingparts is described. An electrode sheet is obtained by placing anelectrode material mixture on both sides of an electrode core member.Specifically, a paste containing an electrode material mixture isapplied to an electrode core member and then dried, whereby theelectrode material mixture can be carried on the electrode core member.

The paste containing an electrode material mixture can be obtained bydispersing an electrode material mixture in a liquid component. Theelectrode material mixture comprises an active material as an essentialcomponent and contains a binder, a conductive agent, etc., as optionalcomponents. As the liquid component, an appropriate one is selecteddepending on the composition of the electrode material mixture. Forexample, an electrode material mixture comprising a carbon material asan active material is preferably dispersed in water,N-methyl-2-pyrrolidone, cyclohexanone, or the like. An electrodematerial mixture comprising a lithium-containing composite oxide as anactive material is preferably dispersed in N-methyl-2-pyrrolidone,cyclohexanone, or the like.

The method by which the paste containing an electrode material mixtureis applied to the surface of the electrode core member is not to beparticularly limited, but it is preferred to intermittently apply thepaste containing an electrode material mixture on both sides of theelectrode core member excluding lead-forming parts. For example, amethod of intermittently applying the paste to the electrode core memberby means of a die coater is commonly employed. Also, after the maskingof the lead-forming parts of the electrode core member with tape, thepaste containing an electrode material mixture may be applied to theelectrode core member, followed by removal of the tape.

Also, the paste containing an electrode material mixture may be appliedto the outer surface of a gravure roll, the paste applied to the outersurface of the gravure roll may be transferred to a surface of anelectrode core member that is being transported by a plurality of guiderolls, and parts of the electrode material mixture may be peeled fromthe core member in order to form lead-forming parts. Even if a smallamount of the electrode material mixture is left on the electrode coremember, there is no particular problem, since the electrode materialmixture is conductive.

Next, the step B of intermittently forming porous insulating layerscomprising an inorganic oxide filler and a binder on a surface of theelectrode sheet excluding the lead-forming parts is described.

The porous insulating layers are obtained by applying a slurrycontaining an inorganic oxide filler and a binder on the surface of theelectrode sheet excluding the lead-forming parts by means of anapplication device including a gravure roll and drying it.

The slurry containing an inorganic oxide filler and a binder can beobtained by dispersing an inorganic oxide filler and a binder in aliquid component. As the liquid component, an appropriate one isselected depending on the kind of the inorganic oxide filler and thebinder, and water, N-methyl-2-pyrrolidone, cyclohexanone, etc., can beused preferably.

FIG. 1 is a schematic sectional view showing an exemplary applicationdevice including a gravure roll. This application device is equippedwith a first guide roll 11 and a second guide roll 12 that are disposedin parallel at a certain interval, and a gravure roll 13 that isdisposed below the guide rolls 11 and 12 and between the guide rolls 11and 12. The whole outer surface of the gravure roll 13 has a gravurepattern engraved thereon. The gravure pattern and the radius of thegravure roll are not to be particularly limited. The first guide roll 11and the second guide roll 12 perform the function of transporting anelectrode sheet 14 in one direction. In FIG. 1, two guide rolls areused, but the number of guide rolls is not to be particularly limited.

Under the gravure roll 13 is a slurry vessel 16 filled with a slurry 15containing an inorganic oxide filler and a binder. The lower outersurface of the gravure roll 13 is positioned below the liquid level ofthe slurry 15. When the gravure roll 13 rotates, the slurry 15 isapplied to the outer surface of the gravure roll 13, and a part of theslurry 15 is scraped off by a blade 17. The blade 17 is preferablycomposed of a resin such as polystyrene, polyethylene, or polypropylene.

The slurry left on the outer surface of the gravure roll 13 is thentransferred to the lower surface of the electrode sheet 14 that is beingtransported by the guide rolls 11 and 12. In FIG. 1, when the electrodesheet 14 passes through the midpoint between the first guide roll 11 andthe second guide roll 12, the lower surface of the electrode sheet 14comes into contact with the upper outer surface of the gravure roll 13,but the positional relation between the electrode sheet 14 and thegravure roll 13 is not to be particularly limited.

For the lead-forming parts, the operation of making the electrode sheetaway from the gravure roll is performed by moving at least one selectedfrom the gravure roll and the guide rolls. For example, the electrodesheet 14 can be made away from the gravure roll 13 by moving the firstguide roll in the direction of arrow A of FIG. 1, moving the secondguide roll in the direction of arrow B, or simultaneously moving thefirst guide roll and the second guide roll in the directions of arrow Aand arrow B. Also, the electrode sheet 14 can be made away from thegravure roll 13 by moving the gravure roll 13 itself in the direction ofarrow C of FIG. 1. At the end position of each lead-forming part, theelectrode sheet 14 and the gravure roll 13 are brought into contact witheach other again.

In the field of conventional gravure printing, the above-mentionedoperation of moving the guide roll or gravure roll upward or downward isnot performed during application. This is because a predeterminedpattern is usually formed on the outer surface of a gravure roll inorder to provide a film having a pattern.

While the above-mentioned operation may be performed manually, it ispreferred to use a computer for control. If a computer is used forcontrol, it is preferred to constantly monitor the surface of theelectrode sheet. At the instant when the beginning position of eachlead-forming part reaches the contact point between the gravure roll andthe electrode sheet, the guide roll(s) and/or the gravure roll are/ismoved upward or downward to make the electrode sheet away from thegravure roll. At the instant when the end position of each lead-formingpart reaches the position at which it is to contact the gravure roll,the guide roll(s) and/or the gravure roll are/is moved back to theoriginal position.

It is preferred that at least a part of the outer surface of the gravureroll, preferably the whole surface thereon, be covered with ceramic. Bycovering the outer surface of the roll with ceramic, the wear of theroll can be prevented, and in addition, the inclusion of foreign matter,such as metal, into the applied film of slurry or paste and a decreasein the thickness accuracy of the applied film can be prevented. As aresult, the life of the application device is improved. The material ofthe ceramic is preferably aluminum oxide (alumina), chromium oxide(chromia), or the like, since they produce a large effect in preventingroll wear.

In terms of uniform application of slurry or paste to the surface of theelectrode sheet or electrode core member, it is preferred that thetraveling direction of the outer surface of the gravure roll be oppositeto the traveling direction of the electrode sheet or electrode coremember, as illustrated in the arrows of FIG. 1.

The transportation speed of the electrode sheet or electrode core memberis not to be particularly limited, but it is preferably 3 to 50 m/min.Also, in terms of uniform application of slurry or paste to the surfaceof the electrode sheet or electrode core member, it is preferred thatthe rotation speed of the outer surface of the gravure roll be set to110 to 250% of the transportation speed of the electrode sheet orelectrode core member.

It is preferred that the slurry containing an inorganic oxide filler anda binder be a Newtonian fluid whose viscosity is 10 to 80 mPa·s at 25°C.

Next, the step C of connecting a lead to each of the lead-forming partsand the step D of fabricating a battery by using the electrode sheet towhich the lead is connected are described.

The connection method of the lead to each of the lead-forming parts isnot to be particularly limited, but welding is commonly performed. Sincethe inorganic oxide filler and the resin binder contained in the porousinsulating layer are not adhering to the lead-forming parts, the weldingis done smoothly, and the contact resistance between the lead and theelectrode core member also becomes small.

In the present invention, the steps A to C are applicable to theproduction of positive electrodes or applicable to the production ofnegative electrodes. Also, the steps A to C are applicable to theproduction of both positive electrodes and negative electrodes. Theporous insulating layer may be formed on the surface of one of theelectrodes so as to be interposed between the positive electrode and thenegative electrode. When the porous insulating layer has a sufficientthickness, a separator sheet may be unnecessary; however, a separatorsheet is usually interposed between the positive electrode and thenegative electrode to form an electrode group.

The electrode group is accommodated with a non-aqueous electrolyte in abattery case. Although the lead connected to each electrode is oftenconnected to a predetermined position of the battery case or a sealingplate that seals the battery case, the connecting position depends onthe kind of the battery. Thereafter, through a usual predetermined step,the battery is completed.

It should be noted that the step A can also use the same applicationdevice as FIG. 1 and perform the same operation as that of the step B,in order to intermittently apply the paste containing an electrodematerial mixture on both sides of the electrode core member excludingthe lead-forming parts. Specifically, the step A can perform thefollowing operation: the paste containing an electrode material mixtureis applied to the outer surface of the gravure roll; the applied pasteis transferred to a surface of the electrode core member that is beingtransported by the plurality of guide rolls excluding at least thelead-forming parts; and for the lead-forming parts, the electrode coremember is made away from the gravure roll by moving at least oneselected from the gravure roll and the guide rolls.

Next, specific structures of lithium ion secondary batteries that can beobtained by the production method of the present invention aredescribed.

The positive electrode sheet includes a positive electrode core memberand a positive electrode material mixture carried on both sides of thepositive electrode core member. Also, the negative electrode sheetincludes a negative electrode core member and a negative electrodematerial mixture carried on both sides of the negative electrode coremember. As the positive electrode core member, for example, aluminumfoil or aluminum alloy foil is used preferably, but there is noparticular limitation. Also, as the negative electrode core member, forexample, copper foil or copper alloy foil is preferably used, but thereis no particular limitation.

The positive electrode active material, which is an essential componentof the positive electrode material mixture, is not to be particularlylimited. However, composite lithium oxides, for example,lithium-containing transition metal oxides, such as lithium cobaltate,lithium nickelate, and lithium manganate, are preferably used. Alsopreferably used are modified lithium-containing transition metal oxidesin which part of the transition metal is replaced with another element.For example, it is preferred that the cobalt contained in lithiumcobaltate be replaced with aluminum, magnesium, or the like, and it ispreferred that the nickel contained in lithium nickelate be replacedwith cobalt, manganese, or the like. These composite lithium oxides maybe used singly or in combination with two or more of them.

The negative electrode active material, which is an essential componentof the negative electrode material mixture, is not to be particularlylimited. However, for example, carbon materials such as natural graphiteand artificial graphite, metal materials such as silicon and tin, alloymaterials such as silicon alloys and tin alloys, etc., are preferablyused. Also usable are vapor-phase growth carbon fibers (VGCF), whichhave high conductivity but increase the asperities of the electrodesurface. These materials may be used singly or in combination with twoor more of them.

Preferable examples of the binder, which is an optional component of thepositive electrode material mixture or negative electrode materialmixture, include fluorocarbon resins such as polytetrafluoroethylene(PTFE) and polyvinylidene fluoride (PVDF), rubber particles such asstyrene butadiene rubber (SBR) and modified acrylonitrile rubber (e.g.,BM-500B available from Zeon Corporation), and soluble modifiedacrylonitrile rubber (e.g., BM-720H available from Zeon Corporation).They may be used singly or in combination with two or more of them.

Preferable examples of the thickener, which is an optional component ofthe positive electrode material mixture or negative electrode materialmixture, include carboxymethyl cellulose (CMC) and polyethylene oxide(PEO). Preferable examples of the conductive agent, which is also anoptional component, include acetylene black, ketjen black, and variousgraphites. They may be used singly or in combination with two or more ofthem. It should be noted that the use of a thickener is particularlypreferable when rubber particles are used as the binder.

The resin binder to be contained in the porous insulating layer is notto be particularly limited, but usable examples include polyacrylic acidderivatives, polyacrylonitrile derivatives, polyvinylidene fluoride(PVDF), polyethylene, styrene-butadiene rubber, polytetrafluoroethylene(PTFE), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP).They may be used singly or in combination with two or more of them.Among them, polyacrylic acid derivatives and polyacrylonitrilederivatives are particularly preferred. It is preferred that thesederivatives contain at least one selected from the group consisting of amethyl acrylate unit, an ethyl acrylate unit, a methyl methacrylateunit, and an ethyl methacrylate unit, in addition to the acrylic acidunit or/and the acrylonitrile unit.

Also, from the viewpoint of enhancing the performance of the negativeelectrode, most negative electrode material mixtures contain rubberparticles such as SBR as a binder and contain a water-soluble resin suchas cellulose resin as a thickener. Thus, when the porous insulatinglayer is formed on the surface of the negative electrode, the resinbinder to be contained in the porous insulating layer is preferablywater-insoluble. This is to prevent the thickener from swelling in thenegative electrode material mixture and to prevent the deformation ofthe negative electrode and hence the decline in yields.

Also, with respect to the resin binder to be contained in the porousinsulating layer, its decomposition-start temperature is preferably 250°C. or higher. This is because in nail penetration tests the temperaturedue to the heat generated upon an internal short-circuit may locallyexceed several hundreds ° C. Also, when the resin binder has a crystalmelting point, the crystal melting point is preferably 250° C. orhigher.

Usable examples of the inorganic oxide filler include titanium oxide,aluminum oxide, zirconium oxide, tungsten oxide, zinc oxide, magnesiumoxide, and silicon oxide. They may be used singly or in combination withtwo or more of them. Among them, particularly in terms of chemicalstability, aluminum oxide (alumina) is preferred, and a-alumina isparticularly preferred.

The inorganic oxide filler is not to be particularly limited, and commonpowder or particulate matter, which comprises primary particles orsecondary particles formed of primary particles that agglomerate by vander Waals force, can be used. Also, an inorganic filler containingparticles of indefinite shape that comprise a plurality of primaryparticles (for example, 2 to 10 particles, preferably 3 to 5 particles)that are joined and adhered to one another can be preferably used.

In the porous insulating layer, the ratio of the resin binder to thetotal of the inorganic oxide filler and the resin binder is not to beparticularly limited. The ratio is, for example, 1 to 50% by weight,preferably 1 to 10% by weight, and more preferably 2 to 5% by weight. Ifthe ratio of the resin binder exceeds 50% by weight, it becomesdifficult to control the porous structure formed by the gaps between theparticles of the inorganic oxide filler. If it is less than 1% byweight, the adhesion of the porous insulating layer to the electrodesurface decreases.

When the separator sheet is not used, the thickness of the porousinsulating layer is preferably, for example, 1 to 20 μm, and morepreferably 3 to 15 μm. Also, when the separator sheet is used, thethickness of the porous insulating layer is preferably 0.5 to 20 μm, andmore preferably 2 to 10 μm. Further, the total of the thickness of theseparator sheet and the thickness of the porous insulating layer ispreferably 15 to 30 μm, and more preferably 18 to 26 μm.

As the separator sheet, a microporous film obtained by molding a resinor resin composition into a sheet form and further drawing the sheet ispreferably used. The resin that is a raw material of the separator isnot to be particularly limited. Polyolefin resins, for example,polyethylene and polypropylene, are often used, but polyamide,polyethylene terephthalate (PET), polyamide imide, polyimide, and thelike are also used. The thickness of the separator sheet is preferably10 to 25 μm.

As the non-aqueous electrolyte, one comprising a non-aqueous solvent anda lithium salt dissolved therein is preferably used.

The non-aqueous solvent is not to be particularly limited, but usableexamples include carbonic acid esters such as ethylene carbonate (EC),propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate(DEC), and ethyl methyl carbonate (EMC); carboxylic acid esters such asγ-butyrolactone, γ-valerolactone, methyl formate, methyl acetate, andmethyl propionate; and ethers such as dimethyl ether, diethyl ether, andtetrahydrofuran. These non-aqueous solvents may be used singly or incombination with two or more of them. Among them, particularly carbonicacid ester is preferably used.

The lithium salt is not to be particularly limited, but preferableexamples include LiPF₆ and LiBF₄. They may be used singly or incombination.

In order to ensure stability upon overcharge, it is preferred that theelectrolyte preferably contain a small amount of an additive that formsa good film on the positive electrode and/or the negative electrode, forexample, vinylene carbonate (VC), vinyl ethylene carbonate (VEC),cyclohexyl benzene (CHB), etc.

The present invention is hereinafter described specifically by ways ofExamples, but the present invention is not to be limited to thefollowing Examples.

Comparative Example 1 (i) Positive Electrode Preparation

A positive electrode material mixture paste was prepared by stirring 3kg of lithium cobaltate with a mean particle size of 3 μm, 1 kg of asolution of polyvinylidene fluoride (#1320 available from KurehaChemical Industry Co., Ltd.) in N-methyl-2-pyrrolidone (NMP) (solidcontent: 12% by weight), 90 g of acetylene black, and a suitable amountof NMP with a double-arm kneader.

Using an application device as illustrated in FIG. 1, the positiveelectrode material mixture paste was applied to both sides of a 15μm-thick aluminum foil sheet, which was a core member (positiveelectrode current collector), and was then dried to produce a positiveelectrode sheet.

The application device used was equipped with a gravure roll havinggrooves at pitches of 20 lines/inch on the outer surface thereof, thegrooves being tilted at 45° C. relative to the rotation axis. Therotation speed of the outer surface of the gravure roll was set to 150%of the transportation speed of the aluminum foil. The travelingdirection of the outer surface of the gravure roll was made opposite tothe traveling direction of the aluminum foil. The whole outer surface ofthe gravure roll used was covered with ceramic made of chromium oxide.

The positive electrode sheet was then rolled such that the totalthickness was 160 μm. Thereafter, predetermined parts of the rolledpositive electrode sheet were sufficiently wiped off with a textilewaste moistened with ethanol, to form exposed parts of the core member,which served as lead-forming parts. A positive electrode lead was weldedto each of the lead-forming parts.

(ii) Negative Electrode Preparation

A negative electrode material mixture paste was prepared by stirring 3kg of artificial graphite with a mean particle size of 20 μm, 75 g of astyrene-butadiene copolymer dispersion (BM-400B available from ZeonCorporation) (solid content: 40% by weight), 30 g of carboxymethylcellulose (CMC), and a suitable amount of water with a double-armkneader.

Using the same application device as that used for the preparation ofthe positive electrode sheet, the negative electrode material mixturepaste was applied in the same manner to both sides of a 10 μm-thickcopper foil sheet, which was a core member (negative electrode currentcollector), and was then dried to produce a negative electrode sheet.

The negative electrode sheet was then rolled such that the totalthickness was 180 μm. Thereafter, predetermined parts of the rollednegative electrode sheet were sufficiently wiped off with a textilewaste moistened with ethanol, to form exposed parts of the core member,which served as lead-forming parts. A negative electrode lead was weldedto each of the lead-forming parts.

FIG. 2 is a schematic sectional view of a negative electrode sheet 20.

Negative electrode material mixtures 22 are carried on both sides of acopper foil sheet 21 serving as a core member. Exposed parts 23 of thecore member are formed by peeling the negative electrode materialmixtures such that there is a location displacement between one side ofthe copper foil 21 and the other side thereof. One of the exposed parts23 of the core member serves as a lead-forming part, to which a lead 24is welded.

Thereafter, the positive electrode sheet and the negative electrodesheet were cut so as to have a width such that they can be inserted intoa cylindrical battery case of size 18650, to obtain a positive electrodeand a negative electrode. The positive electrode and the negativeelectrode were wound, with a separator made of a 20 μm-thickpolyethylene microporous film interposed therebetween, to fabricate anelectrode group. The electrode group was inserted with a non-aqueouselectrolyte into a battery can (battery case).

The electrolyte used was prepared by dissolving LiPF₆ at a concentrationof 1 mol/L in a solvent mixture of ethylene carbonate (EC), dimethylcarbonate (DMC), and ethyl methyl carbonate (EMC) in a weight ratio of2:2:5, and 3% by weight of vinylene carbonate (VC) relative to thiselectrolyte was added thereto. Thereafter, the battery can was sealed,to complete a cylindrical lithium ion secondary battery of size 18650.

Comparative Example 2

A slurry containing an inorganic oxide filler and a resin binder wasprepared by stirring 970 g of alumina having a volume basis meanparticle size (median diameter) of 0.3 μm, 375 g of a polyacrylonitrilederivative dispersion (BM-720H available from Zeon Corporation) (solidcontent: 8% by weight), and a suitable amount of NMP with a double-armkneader.

This slurry was intermittently applied by die coating to one surface ofa negative electrode sheet prepared in the same manner as in ComparativeExample 1 so as to exclude the lead-forming parts, and it was then driedto form 20 μm-thick porous insulating layers. Likewise, porousinsulating layers were then formed on the other surface of the negativeelectrode sheet. In the die coating, the distance between the die nozzleand the surface of the negative electrode sheet was controlled atapproximately 50 μm.

Subsequently, a negative electrode lead was welded to the lead-formingparts. A cylindrical lithium ion secondary battery of size 18650 wascompleted in the same manner as in Comparative Example 1 except for theuse of the negative electrode sheet thus obtained.

FIG. 3 is a schematic sectional view of a negative electrode sheet 30with porous insulating layers obtained in this comparative example.

Negative electrode material mixtures 32 are carried on both sides of acopper foil sheet 31 serving as a core member. Exposed parts 33 of thecore member are formed by peeling the negative electrode materialmixtures such that there is a location displacement between one side ofthe copper foil 31 and the other side thereof. One of the exposed parts33 of the core member serves as a lead-forming part, to which a lead 34is welded. Porous insulating layers 35 are formed so as to cover thewhole negative electrode material mixtures 32. No porous insulatinglayer was provided on the backside of the lead-forming part to which thelead 34 was welded for the purpose of ease of lead connection.

Comparative Example 3

Using an application device as illustrated in FIG. 1, the same slurrycontaining the inorganic oxide filler and the resin binder as that ofComparative Example 2 was continuously applied to one surface of anegative electrode sheet prepared in the same manner as in ComparativeExample 1, and it was then dried to form a 20 μm-thick porous insulatinglayer. Likewise, another porous insulating layer was formed on the othersurface of the negative electrode sheet.

The application device used was equipped with a gravure roll havinggrooves at pitches of 100 lines/inch on the outer surface thereof, thegrooves being tilted at 45° C. relative to the rotation axis. Therotation speed of the outer surface of the gravure roll was set to 150%of the transportation speed of the negative electrode sheet. Thetransportation speed of the negative electrode sheet by the guide rollswas set to 10 m/min. The traveling direction of the outer surface of thegravure roll was made opposite to the traveling direction of thenegative electrode sheet. The whole outer surface of the gravure rollused was covered with ceramic made of chromium oxide.

The predetermined parts of the negative electrode sheet having noelectrode material mixture were sufficiently wiped off with a textilewaste moistened with ethanol, to remove the porous insulating layer. Asa result, exposed parts of the core member serving as lead-forming partswere formed.

Subsequently, a negative electrode lead was welded to each of thelead-forming parts. A cylindrical lithium ion secondary battery of size18650 was completed in the same manner as in Comparative Example 1except for the use of the negative electrode sheet thus obtained.

Example 1

Using an application device as illustrated in FIG. 1, a slurrycontaining an inorganic oxide filler and a resin binder prepared in thesame manner as in Comparative Example 2 was intermittently applied toone surface of a negative electrode sheet prepared in the same manner asin Comparative Example 1 so as to exclude the lead-forming parts, and itwas then dried to form 20 μm-thick porous insulating layers. Likewise,porous insulating layers were formed on the other surface of thenegative electrode sheet.

The application device used was equipped with a gravure roll havinggrooves at pitches of 100 lines/inch on the outer surface thereof, thegrooves being tilted at 45° C. The rotation speed of the outer surfaceof the gravure roll was set to 150% of the transportation speed of thenegative electrode sheet. The transportation speed of the negativeelectrode sheet by the guide rolls was set to 10 m/min. The travelingdirection of the outer surface of the gravure roll was made opposite tothe traveling direction of the negative electrode sheet. The whole outersurface of the gravure roll used was covered with ceramic made ofchromium oxide.

In order to form exposed parts of the core member at the lead-formingparts, the negative electrode sheet was made away from the gravure rollby moving the guide roll upward located forward in the travelingdirection of the negative electrode sheet (corresponding to the firstguide roll 11 of FIG. 1). The movement of the guide roll was controlledby a computer.

Subsequently, a negative electrode lead was welded to each of thelead-forming parts. The negative electrode sheet with the porousinsulating layers obtained in this example has a section that is thesame as that of FIG. 3. A cylindrical lithium ion secondary battery ofsize 18650 was completed in the same manner as in Comparative Example 1except for the use of the negative electrode sheet thus obtained.

Example 2

In the same manner as in Example 1, a slurry containing an inorganicoxide filler and a resin binder was intermittently applied to onesurface of a negative electrode sheet so as to exclude the lead-formingparts, and it was then dried to form 20 μm-thick porous insulatinglayers. On the other hand, a porous insulating layer was formed on thewhole of the other surface of the negative electrode sheet by continuousapplication.

Subsequently, a negative electrode lead was welded to each of thelead-forming parts. A cylindrical lithium ion secondary battery of size18650 was completed in the same manner as in Comparative Example 1except for the use of the negative electrode sheet thus obtained.

FIG. 4 is a schematic sectional view of a negative electrode sheet 40with porous insulating layers obtained in this example.

Negative electrode material mixtures 42 are carried on both sides of acopper foil sheet 41 serving as a core member. Exposed parts 43 of thecore member are formed by peeling the negative electrode materialmixtures such that there is a location displacement between one side ofthe copper foil 41 and the other side thereof. One of the exposed parts43 of the core member serves as a lead-forming part, to which a lead 44is welded. Porous insulating layers 45 are formed so as to cover thewhole negative electrode material mixtures 42. The porous insulatinglayer is also provided on the backside of the lead-forming part to whichthe lead 44 was welded.

Example 3

A slurry containing an inorganic oxide filler and a resin binder wasintermittently applied to the surfaces of a negative electrode sheet soas to exclude the lead-forming parts and was then dried to form 20μm-thick porous insulating layers in the same manner as in Example 1,except that the traveling direction of the outer surface of the gravureroll was made the same as the traveling direction of the negativeelectrode sheet.

Subsequently, a negative electrode lead was welded to each of thelead-forming parts. A cylindrical lithium ion secondary battery of size18650 was completed in the same manner as in Comparative Example 1except for the use of the negative electrode sheet thus obtained.

[Evaluation]

The batteries of Examples 1 to 3 and Comparative Examples 1 to 3 wereevaluated in the following manner. Table 1 shows the result.

(Battery Resistance)

Each battery was subjected to a break-in charge/discharge twice and thenmeasured for its impedance at a measurement frequency of 1 kHz.

(Cycle Capacity Retention Rate)

In an environment of 20° C., each battery was charged and discharged 500cycles in the following patterns (1) to (3), and the ratio of thedischarge capacity at the 500th cycle to the initial capacity wasexpressed in percentage.

(1) Constant current charge: 1400 mA (cut-off voltage 4.2 V)

(2) Constant voltage charge: 4.2 V (cut-off current 100 mA)

(3) Constant current discharge: 400 mA (cut-off voltage 3V)

(Nail Penetration Test)

In an environment of 20° C., each battery was charged in the followingmanner.

(1) Constant current charge: 1400 mA (cut-off voltage 4.25 V)

(2) Constant voltage charge: 4.25 v (cut-off current 100 mA)

In an environment of 20° C., a round iron nail of 2.7 mm in diameter wascaused to penetrate the charged battery from a side face thereof at aspeed of 5 mm/sec. After 1 second, the temperature of the battery nearthe penetration site was measured.

TABLE 1 Temperature Cycle capacity in nail Battery retention ratepenetration resistance (%) test (° C.) Comparative example 1 45 92 146Comparative example 2 44 65 83 Comparative example 3 55 71 74 Example 144 91 77 Example 2 46 90 75 Example 3 45 81 85

[Consideration]

First, in the case of Comparative Example 1 having no porous insulatinglayer, the temperature after 1 second in the nail penetration test washigh, indicating remarkable heating.

Also, in the case of Comparative Example 3, in which after the formationof the porous insulating layer on the whole surface of the negativeelectrode sheet using the gravure roll, a part of the porous insulatinglayer was peeled and the lead was then welded to the core member, thetemperature in the nail penetration test was lower than that ofComparative Example 1, indicating improved safety; however, the batteryresistance was high, and the cycle capacity retention rate was alsoinferior. When the lead was peeled off the core member and thelead-forming part was analyzed, it was found that some of the inorganicoxide filler could not have been removed and remained between the leadand the core member.

In Comparative Example 2, in which the porous insulating layer wasformed by intermittent application by die coating, the temperature inthe nail penetration test was high, though not so high as in ComparativeExample 1, and the cycle capacity retention rate was also significantlyinferior. When the porous insulating layer of Comparative Example 2 wasobserved, it was found that near the lead-forming part the porousinsulating layer was streaked, and that a part of the negative electrodematerial mixture was exposed without being covered with the porousinsulating layer. This is considered to be the reason why thetemperature in the nail penetration test increased and the cyclecapacity retention rate lowered due to uneven charge and dischargereactions.

On the other hand, in Examples 1 and 2, in which the negative electrodesheet wad made away from the gravure roll to expose the core member inthe lead-forming part, the temperature in the nail penetration test islow, and the cycle capacity retention rate is also equivalent to that ofComparative Example 1. This indicates that its safety is sufficientlyenhanced by the porous insulating layer, and that the contact resistancebetween the lead and the core member is maintained low. Also, theproduction yields of the lithium ion secondary batteries of Examples 1and 2 were greatly improved. In Example 3, since the evenness of theporous insulating layer was slightly inferior to those of Examples 1 and2, the temperature in the nail penetration test became a little higherthan those of Examples 1 and 2, and the cycle capacity retention ratelowered a little.

INDUSTRIAL APPLICABILITY

The present invention permits effective production of lithium ionsecondary batteries in which a porous insulating layer is carried on thesurface of an electrode in order to improve the safety againstshort-circuits. It is effective in intermittently forming porousinsulating layers on the surface of an electrode sheet. The presentinvention is useful as a method for producing lithium ion secondarybatteries for use, for example, as the power source for portable powersupply devices that are required to have a high degree of safety andhigh performance.

1-8. (canceled)
 9. An application device for applying a slurry comprising an inorganic oxide filler and a binder to a surface of an electrode plate sheet, comprising: a slurry vessel for containing said slurry; a gravure roll with a gravure pattern engraved on an outer surface thereof for transferring said slurry applied to said outer surface to said surface of said electrode plate sheet, at least a part of said outer surface comprising a ceramic layer comprising aluminum oxide or chromium oxide, said gravure roll being disposed so that said outer surface at the lower side is inside said slurry vessel, and set so that rotation is enabled; a blade comprising polystyrene, polyethylene, or polypropylene disposed so as to abut on said outer surface of said gravure roll, for scraping off a part of said slurry applied to said outer surface; and a first guide roll and a second guide roll for feeding said electrode plate sheet, said first and second guide rolls being disposed above said gravure roll and in a parallel at a certain interval so that said surface of said electrode plate sheet comes into contact with said outer surface at the upper side of said gravure roll and being set so that rotation is enabled; and at least one of said gravure roll, said first guide roll and said second guide roll is capable of reciprocating.
 10. The application device in accordance with claim 9, wherein said surface of said electrode plate sheet and said outer surface at the upper side of said gravure roll are capable of moving away from one another, by at least one of a downward movement of said gravure roll, an upward movement of said first guide roll, and an upward movement of said second guide roll.
 11. The application device in accordance with claim 9, wherein said gravure roll is disposed between said first guide roll and said second guide roll. 